I have been putting a lot of thought into the return system on these motors, specifically the return line to the fuel filter head. I understand that the concept here is to return any excess fuel from the injectors back to the fuel system. So in my mind the fuel would not have excessive pressure resulting in the fuel travelling down the back of the motor back to the tanks and not up the fuel line into the filter head due to gravity. Would this process not create a vacuum in the line from the filter head to the rest of the system that would pull fuel out of the top of the filter and back to the tank when the motor is shut down resulting in a small air gap at the top of the fuel filter every time the truck is shut down? Someone call me out on my thinking if I am wrong but from what I found looking under the hood of my truck this is what I can determine. I feel like the fuel system that goes between injectors acts as a master stream of water like in firefighting and the fuel filter becomes you foam canister, the main stream of fuel creates a venturi as it goes back to the tanks and then draws fuel out of the filter. Obviously with the motor on the lift pump supplies enough fuel to keep the filter flow but the remaining fuel in the system upon shutdown would create this vacuum to draw out fuel from the filter. Maybe I'm just over complicating this but that's how it worked out in my head.

 

Words....socomplicated...
pictures....worth a thousand of ‘em.




The hard line feeds the injector pump.
the soft line returns the fuel to the source.k
its a loop. Easy.

 

But there are 2 exits to that loop

 

On another thread, like yesterday, LW&tjc said it <return line> has a check valve <or something> built in.

into the part the hose connects to at the filter assembly.

 

OK the lift pump is high volume Low pressure excess pressure on the filter head is dumped back to the tanks via the Check Valve on newer heads or via orifice on older style.

The IP passes a lot of fuel mainly to keep it Cool. the Transfer pump inside the IP under the Inlet Cover can pull as much as 15" Hg and make over 90 psi of pressure.

The return lines in the tanks have Check valves too called Duck Bills

Fuel should not flow from the return to the filter head unless you have a Blockage....

diesel Foams so a bit of air is normal......




Drain Back is generally caused by Leaks, IF there are no leaks, then the system works like filling a straw and holding your thumb over the end ..surface tension keeps it from draining out.

The Duck Bills on the return in the tank help keep fuel in the horizontal lines as it takes a small but not massive pressure to make fuel flow thru it.

 

Also I might add that the Housing Pressure regulator (the fitting on the IP that the return line is connected to is also a Check Valve and also takes a small amount of pressure to open it in the direction of flow.

Oh and on the newer filter heads you will hardly ever see fuel flow to the filter unless the check valve is stuck open ... or just plane bad

There are aftermarket return check valves available IF you choose to add them.

 

Ok, thanks for the info. I also wanted to add that I changed my fuel filter a hour ago and it was drained down to half full after sitting for a day, what's up with that?

 

You must have taken your thumb off the straw, and there is an air leak in the
return system to drain the filter back to the tank. My take on it.

There was service bulletin from Ford some years back, to eliminate the rubber
line from the Filter to the front injector on the passenger side. I didn1 have any
problems with it, but the Hard line to the IP started to leak. I changed it over to
Push Lock rubber hose, this eliminated the Olive problems.

While doing that job, I did the return line from the filter delete.

Charlie

 

I never actually Found that TSB to remove the line but I did find the one to install the check valve in place of the orifice, I have several other TSB's on the subject too.
I wish someone would post the TSB number as I should have a copy of it as I have pretty much all TSB's since 1983 to 1997.

But Yes Charlie you right someone removed the finger from the straw... it only takes a Pinhole.... like a Balloon 1 ***** all gone
Thing is if the Duck Bill is missing then surface tension is easily broken for the lines laying against the frame and will cause Siphon action to happen especially if air is getting in.

Image posted so we know what is being talked about.



When folks put high pressure on the return line .... the valve is easily blown off (or Blown apart) to cause other issues in the tank

 

I mentioned Aftermarket check valves here is an example....
they should Stop any Bleed Back, like if the Duck bills are bad and you don't want to pull the tanks.

BTW always put a Brass Valve stem cap with the rubber seal on the schrader valve when it is not in use.

 

Okiedokie!-) LW. Brass valve stem cap with the rubber seal on the list. Easy!-). Thank You!-)

as for the rest of the stuff....
duck bills, tsb’s, surface tension, bb’s and etc. goodness. I understand its the engineering mindset.

for others its just too much.

Are you willing to come up with an “ideal 6.9 fuel system” setup. Just 1,2,3. I dont care “why” 1,2,3...
although its nice to have it there for reference and for others who get into the “why”.
I’m willing to pay money for a solution like that. Heck, am willing to set up a kit like I have done at other
shops and put em on the market. That’s my training. So,

 

Are you willing to come up with an “ideal 6.9 fuel system” setup.........OK just mount the fuel tank on the Roof, no need for Lift pump no need for any check valves Gravity just works

seriously the system is really tried and true ... the problem is the vehicles are getting old and things like the duck bills and the shower heads go neglected because they are a PIA to work on and no one I know enjoys dropping a fuel tank to maintain it... folks just wait until problems arise and are Forced to fix it

Hey my senders in my tanks on my 93 are flaky as hell, I've been putting off fixing them for better than 10 years now .....sooner or later they will bite me in the Butt.

 

The line from the filter head to injector return is so they system can purge air instead of it having to get forced through the ip.

If you think the line is a weak link then install a check valve. It should remain in place as it has a very good purpose.

It can cause fuel drainback without a check valve. But that's only if your return system isn't air tight, if it isn't you should reseal it imho. Anytime the truck is running that line will have positive flow and it shouldn't affect anything.

 

^^^^^^^^^^^^^^^ Exactly .... self bleeding system. works fine.

it takes some Load off the Lift pump too

The duck bills will actually stop bleed back too as long as they are in place and not eaten up.

Actually IF the duck bills are good even with seeping Return O rings it will not cause a big issue ... unless they are a Major leak

 

So I did the whole return line system across the injectors when I replaced them originally, and I got a good reputable kit. I'll go check to make sure all my lines are tight and get a cover for the schrader valve. Seeing my fuel filter drained down that far tells me there is some kind of siphoning action going on downstream because I can't think of any explanation. Although now that I put all of this together it would explain the ridiculously hard starting action I have no matter what the temperature it is outside. Am I right to first go buy a cap for the valve, make sure all return lines are tight and secure, then drop the tank and check/change the sending unit, and while the sending unit is off clear the lines using compressed air? The most expensive part to that whole process is going to be a new sending unit, but if it fixes my hard starting and stalling issue I would consider it money well spent. Also one more thought, my fuel economy has been hanging around the 11 mpg area, granted it is still in the 40's outside but if there is a blockage somewhere could this be the cause of such low numbers. Everyone else I see on here is getting 14 to 15 and I am baffled as to why I can't get over the 12 mark.